NATO has pledged support for Ukraine's navy more than two weeks after Russia seized three of Kyiv's naval ships and arrested 24 sailors in the Kerch Strait that links the Black Sea with the Sea of Azov.

The alliance has been 'supporting Ukraine to improve its naval capabilities, logistics and cyberdefense,' NATO Secretary-General Jens Stoltenberg told Ukrainian President Petro Poroshenko in Brussels on December 13.

NATO will also deliver secure communications equipment to Ukraine's military by the end of this year, Stoltenberg said, according to an official transcript.

On November 25, the Russian Coastguard opened fire and detained several Ukrainian vessels and 24 crew members in the Kerch Strait.

Russia alleged that the vessels had illegally entered Russian territorial waters near the Crimea region, which Russia occupied and annexed from Ukraine in 2014.

Ukraine and most UN member states do not recognize the annexation.

'Russia must immediately release the sailors and ships they seized and allow freedom of navigation including free access to Ukrainian ports in the Sea of Azov,' Stoltenberg said.